The invention relates to a high-temperature alloy for thermal machines based on intermetallic compounds that are suitable for waste-wax casting and directional solidification and that supplement conventional nickel-based super alloys.
It concerns an improvement of alloys based on an intermetallic compound of the titanium aluminide TiAl type with other additives that increase strength, toughness, and ductility as well as oxidation and creep resistance.
Intermetallic compounds of titanium with aluminum have several interesting properties that make them attractive as construction materials in the intermediate and higher temperature range. This includes their lower density than supper alloys. However, their technical utility in the present form is adversely affected by their brittleness. This can be improved by specific additives.
It has been suggested, for example, to add alternatively Cr, B, V, Si, Ta as well as Mn, W, Mo, Nb, Hf or (Ni+Si) to reduce brittleness on the one hand and to achieve the highest possible strength in the temperature range of interest between room temperature and operating temperature on the other hand. A sufficiently high oxidation resistance also has been desired. These objectives were only partially realized, however.
Especially the heat resistance of known aluminides is falling short of desired values. In accordance with the relatively low fusion point of these materials, the strength, in particular creep resistance, in the upper temperature is insufficient.
U.S. Pat No. 3,203,794 discloses a TiAl high-temperature alloy with 37 wt. % Al, 1 wt. % Zr, and the rest Ti. The relatively small addition of Zr results in this alloy having properties comparable to pure TiAl.
EP-A1-0 363 598 discloses a high-temperature alloy based on TiAl with additives of Si and Nb, while EP-A1-0 405 134 discloses a high-temperature alloy based on TiAl with additives of Si and Cr.
However, these known, modified intermetallic compounds do not fulfill the technical requirements.
In order to improve the properties, EP-B1-0 455 005 therefore disclosed a high-temperature alloy based on doped TiAl and having the following chemical composition:
TixE1yMezAl1xe2x88x92(x+Y+z), whereby
E1=B, Ge or Si and Me=Cr, Mn, Nb, Pd, Ta, W, Y, Zr, and the following applies:
0.46xe2x89xa6xxe2x89xa60.54,
0.001xe2x89xa6y less than 0.015 for E1=Si and Me=W
0.001xe2x89xa6yxe2x89xa60.015 for E1=Ge and Me=Cr, Ta, W
0 less than yxe2x89xa60.02 for E1=Ge and Me=Pd, Y, Zr
0.0001xe2x89xa6yxe2x89xa60.01 for E1=B
0.01 less than zxe2x89xa60.04, if Me=single element,
0.01 less than zxe2x89xa60.08, if Me=two or more single elements
and 0.46xe2x89xa6(x+y+z) less than 0.54.
By adding W, Cr, Mn, Nb, Y, Zr, Pd to the alloy, a higher hardness and strength is achieved than with the TiAl base alloy. The addition of B increases ductility. Si increases oxidation resistance. The range of application for these modified titanium aluminides extends to temperatures between 600xc2x0 C. and 1000xc2x0 C.
Another improvement, especially of creep resistance and oxidation resistance, in the above described alloy is achieved if E1 is in each case a combination of two elements from the group B, Si, and Ge (DE 199 33 633.4).
A high-temperature alloy for a mechanically highly stressed component of a thermal machine has the following composition (in atomic %) based on doped TiAl:
44.5 to less than 46 Al,
1-4 W,
0.1-1.5 Si,
0.0001-4 B, and
Rest Ti and contaminations due to the manufacturing process.
The alloy has an Al content that is lower than in known alloys on the one hand, and, on the other hand, a significantly higher B content.
In one aspect, the combination of the mentioned alloy elements, in particular, however, the higher B contents, makes it possible to produce, on the one hand, a very fine grain both for thin and large cross-sections, and in this way to increase the strength and creep resistance and on the other hand achieve a good oxidation resistance. The reduction of the Al content in comparison to the known state of the art increases strength, but at the same time promotes a larger grain size. Boron in contrast stabilizes the grain limits, i.e., higher boron levels reduce the amount of grain enlargement.
In one embodiment, the high-temperature alloy has the following composition (in atomic %):
44.5 to less than 46 Al,
1-3 W,
0.4-1 Si,
1-4 B, and
Rest Ti and contaminations due to the manufacturing process.
In a further embodiment, the high-temperature alloy has the following composition (in atomic %):
45 Al,
2 W,
0.5 Si,
2 B, and
Rest Ti and contaminations due to the manufacturing process.